package org.apache.lucene.util;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/** A PriorityQueue maintains a partial ordering of its elements such that the
 * least element can always be found in constant time.  Put()'s and pop()'s
 * require log(size) time.
 *
 * <p><b>NOTE</b>: This class pre-allocates a full array of
 * length <code>maxSize+1</code>, in {@link #initialize}.
 * 
 * @lucene.internal
*/
public abstract class PriorityQueue<T> {
    private int size;
    private int maxSize;
    private T[] heap;

    /** Determines the ordering of objects in this priority queue.  Subclasses
     *  must define this one method.
     *  @return <code>true</code> iff parameter <tt>a</tt> is less than parameter <tt>b</tt>.
     */
    protected abstract boolean lessThan(T a, T b);

    /**
     * This method can be overridden by extending classes to return a sentinel
     * object which will be used by {@link #initialize(int)} to fill the queue, so
     * that the code which uses that queue can always assume it's full and only
     * change the top without attempting to insert any new object.<br>
     * 
     * Those sentinel values should always compare worse than any non-sentinel
     * value (i.e., {@link #lessThan} should always favor the
     * non-sentinel values).<br>
     * 
     * By default, this method returns false, which means the queue will not be
     * filled with sentinel values. Otherwise, the value returned will be used to
     * pre-populate the queue. Adds sentinel values to the queue.<br>
     * 
     * If this method is extended to return a non-null value, then the following
     * usage pattern is recommended:
     * 
     * <pre>
     * // extends getSentinelObject() to return a non-null value.
     * PriorityQueue<MyObject> pq = new MyQueue<MyObject>(numHits);
     * // save the 'top' element, which is guaranteed to not be null.
     * MyObject pqTop = pq.top();
     * &lt;...&gt;
     * // now in order to add a new element, which is 'better' than top (after 
     * // you've verified it is better), it is as simple as:
     * pqTop.change().
     * pqTop = pq.updateTop();
     * </pre>
     * 
     * <b>NOTE:</b> if this method returns a non-null value, it will be called by
     * {@link #initialize(int)} {@link #size()} times, relying on a new object to
     * be returned and will not check if it's null again. Therefore you should
     * ensure any call to this method creates a new instance and behaves
     * consistently, e.g., it cannot return null if it previously returned
     * non-null.
     * 
     * @return the sentinel object to use to pre-populate the queue, or null if
     *         sentinel objects are not supported.
     */
    protected T getSentinelObject() {
        return null;
    }

    /** Subclass constructors must call this. */
    @SuppressWarnings("unchecked")
    protected final void initialize(int maxSize) {
        size = 0;
        int heapSize;
        if (0 == maxSize)
            // We allocate 1 extra to avoid if statement in top()
            heapSize = 2;
        else {
            if (maxSize == Integer.MAX_VALUE) {
                // Don't wrap heapSize to -1, in this case, which
                // causes a confusing NegativeArraySizeException.
                // Note that very likely this will simply then hit
                // an OOME, but at least that's more indicative to
                // caller that this values is too big.  We don't +1
                // in this case, but it's very unlikely in practice
                // one will actually insert this many objects into
                // the PQ:
                heapSize = Integer.MAX_VALUE;
            } else {
                // NOTE: we add +1 because all access to heap is
                // 1-based not 0-based.  heap[0] is unused.
                heapSize = maxSize + 1;
            }
        }
        heap = (T[]) new Object[heapSize]; // T is unbounded type, so this unchecked cast works always
        this.maxSize = maxSize;

        // If sentinel objects are supported, populate the queue with them
        T sentinel = getSentinelObject();
        if (sentinel != null) {
            heap[1] = sentinel;
            for (int i = 2; i < heap.length; i++) {
                heap[i] = getSentinelObject();
            }
            size = maxSize;
        }
    }

    /**
     * Adds an Object to a PriorityQueue in log(size) time. If one tries to add
     * more objects than maxSize from initialize an
     * {@link ArrayIndexOutOfBoundsException} is thrown.
     * 
     * @return the new 'top' element in the queue.
     */
    public final T add(T element) {
        size++;
        heap[size] = element;
        upHeap();
        return heap[1];
    }

    /**
     * Adds an Object to a PriorityQueue in log(size) time.
     * It returns the object (if any) that was
     * dropped off the heap because it was full. This can be
     * the given parameter (in case it is smaller than the
     * full heap's minimum, and couldn't be added), or another
     * object that was previously the smallest value in the
     * heap and now has been replaced by a larger one, or null
     * if the queue wasn't yet full with maxSize elements.
     */
    public T insertWithOverflow(T element) {
        if (size < maxSize) {
            add(element);
            return null;
        } else if (size > 0 && !lessThan(element, heap[1])) {
            T ret = heap[1];
            heap[1] = element;
            updateTop();
            return ret;
        } else {
            return element;
        }
    }

    /** Returns the least element of the PriorityQueue in constant time. */
    public final T top() {
        // We don't need to check size here: if maxSize is 0,
        // then heap is length 2 array with both entries null.
        // If size is 0 then heap[1] is already null.
        return heap[1];
    }

    /** Removes and returns the least element of the PriorityQueue in log(size)
    time. */
    public final T pop() {
        if (size > 0) {
            T result = heap[1]; // save first value
            heap[1] = heap[size]; // move last to first
            heap[size] = null; // permit GC of objects
            size--;
            downHeap(); // adjust heap
            return result;
        } else
            return null;
    }

    /**
     * Should be called when the Object at top changes values. Still log(n) worst
     * case, but it's at least twice as fast to
     * 
     * <pre>
     * pq.top().change();
     * pq.updateTop();
     * </pre>
     * 
     * instead of
     * 
     * <pre>
     * o = pq.pop();
     * o.change();
     * pq.push(o);
     * </pre>
     * 
     * @return the new 'top' element.
     */
    public final T updateTop() {
        downHeap();
        return heap[1];
    }

    /** Returns the number of elements currently stored in the PriorityQueue. */
    public final int size() {
        return size;
    }

    /** Removes all entries from the PriorityQueue. */
    public final void clear() {
        for (int i = 0; i <= size; i++) {
            heap[i] = null;
        }
        size = 0;
    }

    private final void upHeap() {
        int i = size;
        T node = heap[i]; // save bottom node
        int j = i >>> 1;
        while (j > 0 && lessThan(node, heap[j])) {
            heap[i] = heap[j]; // shift parents down
            i = j;
            j = j >>> 1;
        }
        heap[i] = node; // install saved node
    }

    private final void downHeap() {
        int i = 1;
        T node = heap[i]; // save top node
        int j = i << 1; // find smaller child
        int k = j + 1;
        if (k <= size && lessThan(heap[k], heap[j])) {
            j = k;
        }
        while (j <= size && lessThan(heap[j], node)) {
            heap[i] = heap[j]; // shift up child
            i = j;
            j = i << 1;
            k = j + 1;
            if (k <= size && lessThan(heap[k], heap[j])) {
                j = k;
            }
        }
        heap[i] = node; // install saved node
    }

    /** This method returns the internal heap array as Object[].
     * @lucene.internal
     */
    protected final Object[] getHeapArray() {
        return (Object[]) heap;
    }
}
